1. Field of the Invention
This invention relates to an optical path rotating device and a laser apparatus using this optical path rotating device. More particularly, this invention relates to a semiconductor laser condenser for condensing semiconductor laser beams into a small spot and a semiconductor-laser-pumped solid state laser apparatus in which solid state laser elements are optically pumped.
2. Description of Related Art
The YAG laser has been used for laser processing or medical applications. However, the YAG laser, which is a solid state laser, has a low electro-optic conversion efficiency. The reason is that only a small portion of emission energy can be used to pump the solid state laser because in the conventional YAG laser, the luminous efficiency of a Xe lamp or a flash lamp used for pumping the solid state laser is low and the spectrum width of light emission is wide. Therefore, the equipment has to be large in size and normally requires cooling water.
On the other hand, the semiconductor laser (LD) has a high conversion efficiency, is compact in size and does not require an extensive cooling system. Recently, the cost of high output semiconductor lasers is decreasing notably. It is desirable to use semiconductor lasers even in the field of laser processing. However, the semiconductor lasers are generally inferior in laser beam quality, and, furthermore, there is a limit to enlarging the output of the single-striped semiconductor laser, so that it is difficult to use the semiconductor laser as is for laser processing. A multi-stripe array semiconductor laser having arranged linearly therein 10 to 100 active layer stripes to serve as a broken-line-shaped light source for emitting laser beams is known as a high output laser.
As for semiconductor lasers of a linear array type having active layer stripes arranged linearly, CW (continuous wave) lasers with high optical power of 20 W are now available on the market. In a multi-stripe array semiconductor laser, as shown in FIG. 1 for example, ten to several tens of 100-200 .mu.m wide stripes having emitters at their ends are arranged at fixed intervals within a flat face of the laser having a total width of about 1 cm.
As described above, one piece of semiconductor laser element provides a light source composed of line segments for emitting ten to several tens of laser beams arranged in line. Some means for condensing a high-level energy in a narrow area should be contrived in order to apply the multi-stripe array semiconductor laser to laser processing or medical applications.
Each stripe beam is emitted from a flat light source, and with reference to the divergence angle of the beam, the vertical component .phi. in relation to the active layer is about as wide as 40 to 50 degrees, while the horizontal component 8 is about as small as 10 degrees. With reference to the widths of each light source, the vertical component is narrow with up to 1 .mu.m, while the horizontal component is wide with 100 to 200 .mu.m as mentioned above.
Due to the characteristics of the semiconductor laser mentioned above, when the emitted beams from the semiconductor lasers are condensed and converged with lenses, the vertical components can be converged easily, but it is difficult to converge the horizontal components into a small spot because the total width of the light sources is wide and the divergence angle of the horizontal components is narrower than of vertical components. D. C. Shannon et. al. disclose in Opt. Lett., 16, 318 ( 1991) an apparatus which uses an aspherical lens to converge the beams of the semiconductor laser array in a spot to pump a solid state laser. However, in this case, there is a great coupling loss of the optical fiber. In addition, the horizontal components of the LD beams cannot be condensed to an area smaller than several millimeters, so that some method needs to be contrived, such as distorting the surface of the solid state laser resonator to match it with the pump space. On the other hand, Yamaguchi et. al. disclose in JP-A-04-078179 (which corresponds to U.S. patent application Ser. No. 07/828,347 by the same assignee of the present patent application) that as shown in FIG. 2, micro lenses are arranged on a one-to-one correspondence with the stripes, and after condensing and collimating the beams from the stripes, this plurality of beams is condensed and superposed on one another by a condenser lens, and by this method, the beams can be converged upon a relatively small area. However, the converged beam spot diameter is a value obtained by multiplying the light source width by the magnification (f.sub.2 /f.sub.1) determined by the ratio of the distance between the condenser lens and the beam spot (i.e., the focal length f.sub.2 of the condenser lens) to the distance between the semiconductor laser stripes and the micro lens array (i.e., the focal length f.sub.1 of the micro lenses). Therefore, the length w.sub.1 (horizontal component) of the beam spot is a value (.omega..theta.f.sub.2 /f.sub.1) obtained by multiplying the width of a stripe (.omega..theta.: 100-200 .mu.m) by the above-mentioned magnification. The vertical component of the beam spot does not amount to a large diameter when the width of the vertical component of the light source is multiplied by the same magnification (f.sub.2 /f.sub.1) because the vertical width is very small (no more than 1 .mu.m). Therefore, in view of the light convergence in the width direction of the stripes, in order to increase the light intensity by reducing the beam spot, it would be better to arrange the micro lenses as distant from the stripes as possible. However, this is difficult to realize because the radiant energy leaking out of the lens apertures located at a distance from the stripes is large owing to the large divergence angle of the vertical components of the stripe beams.
A possible solution is to condense the vertical components and the horizontal components by separate cylindrical lenses and by arranging the lens for condensing the vertical components close to the stripes and the micro cylindrical lenses for condensing the horizontal components remote from the stripes. The micro cylindrical lenses are provided on a one-to-one correspondence with the stripes. A typical LD stripe array product available on the market is one which has twelve stripes having 1 .mu.m thickness and 200 .mu.m width arranged at 800 .mu.m pitches. The adjacent horizontal components of the beams from the stripes, each having a beam divergence angle of 10.degree. overlap each other at about 3.4 mm from the emitter end of the stripes. If the micro lenses are placed beyond this overlapping point, part of the beams travel at a certain angle from the axes of the lenses, and converge at a point different from the focus of the focusing lens, so that the efficiency of the system is reduced. Therefore, in order to collimate the beams from the stripes by using a micro cylindrical lens array, it is necessary to place the lenses at a position less than 3.4 mm away from the stripes (focal distance f.sub.1 .ltoreq.3.4 mm). The converged spot diameter is inevitably large when the spot diameter is estimated by multiplying the stripe width by the magnification (f.sub.2 /f.sub.1) determined by involving the focal distance f.sub.2 of the condenser lens for condensing the collimated beams.
As has been discussed, it has been difficult to focus the laser beams in the form of a broken line emitted from the linear array of LDs in a small area at a high energy density.
According to an end surface pumping method for optically pumping the solid state laser in the direction of its optical axis in a semiconductor-laser-pumped solid state laser, a highly efficient emission of single fundamental transverse mode can be realized by matching the pumping space by a semiconductor laser output light width the mode space of solid state laser oscillation.
A multi-stripe array semiconductor laser element having semiconductor laser active layers arranged in a line produces output power of 10 W or more which is sufficient for laser micro processing. If the multi-stripe laser beams can be converged directly into a sufficiently thin spot by using an optical system, the semiconductor laser output should be able to be directly used for laser beam machining.
However, as described above, when the emitted beams from the semiconductor laser generating element are condensed and converged by lenses, the vertical components of the beams may be converged into a small spot, but it is difficult to converge the horizontal components in small areas since the whole width of the light sources is so large.
When such a linear array semiconductor laser is used as a pumping light source, as the array width reaches around 1 cm, a plurality of light beams cannot be converged into a small spot by an ordinary optical system, and for this reason, the end pumping method which is excellent in pumping efficiency cannot be adopted, and therefore a side surface pumping method could only be used with this linear array semiconductor laser as the light source.